Many applications for optical equipment require relatively uniform radiation across the extent of a target plane. For example, in photolithography, light is projected from a lamp through a mask or transparency at a target plane, behind which a photosensitive medium is disposed. It may be desirable for the projected light to be uniform so that the areas of varying transparency of the mask are truly recorded on the photosensitive medium. Non-uniformities or discontinuities in the light source and/or the reflector can result in poor imaging.
It is also desirable for the radiation at the target plane to be of small local divergence, where local divergence is defined as the solid angle subtended by the source, as seen from points on the target.
It may also be desirable for the radiation which is projected on the target area to be collimated or substantially collimated.
In some prior art photolithography systems, as shown in Japanese laid open Patent Application No. 58-35861, refractive optics are employed to attempt to make the projected light uniform. However, such systems do not have a small local divergence, as the divergence of such systems measured at the center of the target plane is approximately the same relatively large angle as the divergence at the edge of the target. Accordingly, such prior art systems may not achieve the desired resolution at the photosensitive medium.
U.S. Pat. No. 4,683,525 to Camm and assigned to Fusion Systems Corporation, describes a lamp which has a reflector capable of providing the combination of a substantially uniform light flux and a small average local divergence over the target area. However, a lamp which, in addition, provides improved collimation is desired.